Crescent gear pumps are well known and have an outer rotor which is internally toothed and an inner rotor which is externally toothed. The outer rotor has a number of teeth and the inner rotor has fewer teeth, according to known design considerations. The inner and outer rotors are rotatably mounted in a pump housing having an inlet and an outlet and the axes of rotation of the rotors are spaced from one another, with their teeth meshing in a region between the inlet and outlet of the pump housing. A crescent-shaped member is located between the two rotors, opposite where the teeth mesh, and the tips of the teeth of the inner rotor sealingly engage the inner surface of the crescent and the tips of the teeth of the outer rotor sealingly engage the outer surface of the crescent as the rotors rotate to separate the inlet and outlet of the housing to allow the pump to pressurize the working fluid.
The design of the rotor set (i.e.—the inner and outer rotors), and in particular the shape of the gear teeth, for a crescent gear pump is important to ensure proper operation of the pump. Poorly designed rotor sets can suffer from poor and/or inefficient performance, operating noise, output pulsations and other problems. Further, the design of the rotor set must consider the manufacturability of the rotor set.
Previous attempts to provide rotor sets for gear pumps with desired properties have included U.S. Pat. No. 3,907,470 to Harle et al. which teaches forming the teeth of the outer rotor in a substantially trochoidal (i.e.—either completely circular-based or partially hypocycloidal-based) shape and generating the inner rotor. U.S. Pat. No. 4,155,686 to Eisenmann et al. teaches an improvement to the teaching of Harle et al. wherein the profile of the teeth of the generated inner rotor are cut-back from their generated shape to limit the contact areas of the meshing between the teeth of the rotors.
The benefits of using a substantially trochoidal tooth profile include improvements to both noise and displacement. A substantially trochoidal tooth profile enables the number of outer rotor teeth to be smaller than other designs and this results in the tooth gaps of the outer rotor being relatively large. This also results in the corresponding fluid pumping chambers formed between the teeth of the inner and outer rotors and the crescent being large and thus the resulting pumps have a correspondingly large displacement (volumetric capacity). Further, using a substantially trochoidal tooth profile provides a low tooth contact frequency translating into a low frequency operating noise for the pump.
More recently, U.S. Pat. No. 5,163,826 teaches a rotor set for a gear pump wherein the teeth of both the inner and the outer rotors have dual cycloidal profiles formed from epicycloidal and hypocycloidal arcs. This design allows for the rotor set to have an increased displacement in comparison to outer rotor-only trochoidal designs.
While the rotor set designs of the prior art provide reasonable performance, they still suffer from higher levels of operating noise than is desired. Further, the displacement of pumps of a given physical size (i.e. “package size”) employing such rotor sets is less than is desired.